The accurate measurement of surface temperatures of hot objects is of concern in many industrial and scientific processes. For instance, temperatures must be accurately measured and controlled during the fabrication of semiconductor devices. In particular, the temperature of semiconductor wafers must be accurately monitored during rapid thermal processing of the wafers, during rapid thermal oxidation of the wafers, or during other processes which modify or add thin chemical films or coatings to the surface of the wafers. For these semiconductor fabrication processes, the temperature of the substrate should be known within a few degrees over a range which may extend from less than 400.degree. C. to over 1,100.degree. C.
In the past, the temperature of hot objects was determined either using (1) contact methods or (2) non-contact methods. For instance, during contact methods, the hot object is contacted with a sensor such as a thermocouple that is in turn connected to a temperature meter, which indicates the temperature of the object. Conventional non-contact methods of determining temperature, on the other hand, include using a light sensor such as an optical pyrometer that senses the thermal radiation being emitted by the object at a particular wavelength of light. Once the thermal radiation being emitted by the object is known, the temperature of the object can be estimated.
When processing semiconductor materials for use in the electronics industry, it is generally preferable to use non-contact methods when measuring the temperature of the semiconductor wafers. For instance, one advantage of non-contact methods is that the wafer can be rotated during the heating process, which promotes uniform temperature distribution throughout the wafer. Rotating the wafer also promotes more uniform contact between the flow of processing gases and the wafer. Besides being able to rotate the wafers, another advantage to using non-contact methods is that, since no temperature gauges need be attached to the wafer, the wafers can be processed much more quickly saving precious time during semiconductor fabrication.
For all of the high temperature wafer processes of current and foreseeable interest, one of the more important requirements is that the true temperature of the wafer be determined with high accuracy, repeatability and speed. The ability to accurately measure the temperature of a wafer has a direct payoff in the quality and size of the manufactured semiconductor devices. For instance, the smallest feature size required for a given semiconductor device limits the computing speed of the finished microchip. The feature size in turn is linked to the ability to measure and control the temperature of the device during processing. Thus, there is increasing pressure within the semiconductor industry to develop more accurate temperature measurement and control systems.
In this regard, the chief disadvantage of conventional non-contact optical pyrometry systems for determining temperature is that the systems measure an apparent temperature rather than the true temperature of the wafer. In particular, a real surface emits radiation less efficiently than an ideal or perfect blackbody. Through theory and calculation, once the emitted radiation of a blackbody is known, the temperature of the blackbody can be calculated. A real body, however, such as a wafer, emits only a fraction of the radiation that would be emitted by a blackbody at the same temperature. This fraction is defined as the emittance of the real object. Thus, when sensing the radiation being emitted by a real body, a pyrometer generally indicates an apparent temperature that can be different from the true temperature of the object.
Thus, in order to measure the true temperature of a real body using a pyrometer, the indicated temperature must be corrected to account for the emittance. Unfortunately, the emittance of a real body is generally unknown and is very difficult to measure accurately. Further, the emittance of semiconductor wafers varies from wafer to wafer. The emittance is a property of the wafer and depends on several parameters, such as the chemical composition of the wafer, the thickness of the wafer, the surface roughness of the wafer, any coatings present on the wafer and the wavelength at which the pyrometer operates.
In addition to being able to determine the emittance of the semiconductor wafer, problems in accurately determining the temperature of the wafer can also occur when the wafer is semi-transparent at the wavelength at which the pyrometer operates. This problem is especially prevalent at lower temperatures.
In the past, some methods have been proposed for measuring the properties of the semiconductor wafer prior to processing the wafer or during processing of the wafer. For example, U.S. Pat. No. 6,056,434 discloses a method by which the reflectivity of the semiconductor wafer is measured to assist in determining the emittance of the wafer.
The present disclosure is directed to further improvements in methods for determining the optical properties of substrates, such as semiconductor wafers that are to be processed in thermal processing chambers. The properties or characteristics of the wafer that are determined according to the present disclosure may then be used to better control the heating process and/or the manner in which the substrate is heated.